Page 162 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 149

Several disadvantages are related to wireline logging. We already mentioned mud
invasion. Some logging jobs may last several days and as the ‘openhole time’ increases
the quality of acquired data and the stability of the borehole will deteriorate.
Wireline logging is also expensive both in terms of service charges by the logging
company and in terms of rig time. It may therefore be desirable to measure
formation properties whilst drilling is in progress. Not only would this eliminate the
drawbacks of wireline operations but the availability of real time data allows
operational decisions, for example selection of completion intervals, or sidetracking
to be taken at a much earlier stage.

6.3.5. Logging/measurement while drilling (LWD/MWD)

Basic MWD technology was first introduced in the 1980s by drilling companies, and
was initially restricted to retrievable inserts for directional measurements and then
natural GR logs. These developments were quickly followed by logging tools
integrated into drill collars (DCs) (LWD). Recently, LWD development has progressed
to the stage where most of the conventional wireline logging tools can be effectively
replaced by a LWD equivalent. Early LWD technology was often considered to be
inferior to wireline. However, recent mergers between wireline and drilling companies
has resulted in technology-transfer inR&D whichhas ledtoasignificant
improvement in LWD log quality. A lazy use of terminology within the industry
means that LWD and MWD can be considered as synonymous. A more appropriate
term for today’s sophisticated devices is formation evaluation while drilling (FEWD).
Perhaps the greatest stimulus for the development of such tools has been the
proliferation of high-angle wells in which deviation surveys are difficult and wireline
logging services are impossible (without some sort of pipe conveyance system), and
where LWD logging can minimise formation damage by reducing openhole exposure
times.
Whilst providing deviation and logging options in high-angle wells is a
considerable benefit, the greatest advantage offered by LWD technology, in either
conventional or high-angle wells, is the acquisition of real time data at surface. Most
of the LWD applications which are now considered standard, exploit this feature in
some way, and include
real time correlation for picking coring and casing points
real time overpressure detection in exploration wells
real time logging to minimise ‘out of target’ sections (geosteering)
real time formation evaluation to facilitate ‘stop drilling’ decisions.

Although there are a wide range of LWD services available, not all are required
in every situation and the full LWD logging suites which include directional and
formation logging sensors are run much less frequently than gamma/resistivity/
directional combinations. An example of an LWD tool configuration is given in
Figure 6.37.
All LWD tools have both a power supply and data transmission system, often
combined in one purpose-built collar and usually located above the measurement
sensors as shown in Figure 6.38 (a Baker–Hughes multicombination tool).